This invention relates to waveguide junction circulators, and in particular to means for absorbing spurious modes produced therein.
Circulators are used in a variety of microwave transmission systems for interconnecting microwave components with little reflection. One particular type of circulator is the waveguide junction circulator which consists of at least three branches each coupled to a waveguide of the transmission apparatus. These branches converge at a junction wherein there is disposed a nonreciprocal gyromagnetic material, such as a ferrite. A magnetic field is applied to this element so that a wave incident thereon will produce various modes inside and outside the element which cause the energy to be coupled to one of the adjacent branches but not the other. Thus, the microwave signal can be transmitted in only one direction in the circular without undesired reflection in the other direction. If a resistive termination is coupled to one of the branches, the circulator is used as an isolator to prevent reflection of waves from one component to another.
Although the exact nature of the modes produced in a waveguide junction circulator are not well understood, it appears that the incident wave is split by the gyromagnetic element into a set of dielectric modes comprising a linearly polarized in-phase mode, a right circularly polarized mode, and a left circularly polarized mode. The latter two modes have an electric field orthogonal to that of the incident wave and travel the length of the gyromagnetic body, which body has its axis parallel to the electric field of the incident wave. It is the interaction of all three modes which produces the circulator action (for a more detailed discussion, see Owen and Barnes "The Compact Turnstile Circulator," IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-18, No. 12, pages 1096-1100 (December 1970), which is incorporated by reference herein).
It is found, however, that certain higher order, spurious modes which do not contribute to circulator action will also be produced by the gyromagnetic material. These spurious modes produce resonance spikes which often lie in the desired passband frequency of the propagated wave thereby reducing the width of the useable passband and in general adversely affecting electrical characteristics. One approach in dealing with these spurious modes involves choosing the proper length-to-diameter ratio of the gyromagnetic element so that the resonant spikes are moved out of the passband. Because the fields of both the circulator modes and the spurious modes occupy the same space, it is difficult to achieve the necessary differential control to move the spurious responses out of band by this method. Some recent alternative suggestions have been directed toward suppression of such unwanted modes. These usually involve use of a metal ring disposed between stacked ferrite elements (see, for example, U.S. Pat. No. 3,662,291, issued to Cotter. It has also been proposed to suppress unwanted modes by placing a metal ring on top of an open faced gyromagnetic element (see, for example, Tan and Helszajn, "Suppression of Higher Order Modes in Waveguide-Junction Circulators Using Coupled Open Dielectric Resonators," IEEE Transactions on Microwave Theory and Techniques, May 1976, pages 271-273). Such approaches basically seek to detune the spurious modes and thereby move them outside the passband.
It is a primary object of the invention to provide an alternative means for removing spurious modes produced in waveguide junction circulators by absorbing those modes with little or no effect on the propagating wave.